Various viscous fluid couplings have been heretofore proposed. One known viscous fluid coupling is shown in FIG. 6, where a rotor 1 is mounted on a input shaft 2. A fluid chamber 5 consisting of sections 5a and 5b is filled with viscous fluid to transmit torque from the input to the output. The section 5a is formed by the rotor 1 and a case 3. The section 5b is formed by the rotor 1, the case 3, and a cover 4, or output shaft. A reservoir 6 stores the fluid. A member 7 which responds to temperature variations is connected to a valve 9 via a rod 8. The valve 9 is opened or closed by the member 7 via the rod 8, depending on temperature, in order to control the flow of viscous fluid between the fluid chamber 5 and the reservoir 6. Thus, the torque transmitted from the input to the output is controlled.
The above-described known coupling has a disadvantage. In particular, when the coupling is not in operation, the fluid stays in the lower art of the coupling because of gravity. Therefore, the fluid chamber 5 is filled with the fluid, as is the reservoir 6, as shown in FIG. 8. When the coupling is again set into operation, torque is transmitted from the input by the viscous fluid that fills the chamber 5. Consequently, a fan (not shown) rotates at a high speed until the fluid is pumped back into the reservoir 6 from the chamber 5. In this case, if the ambient temperature is sufficiently high, and if the valve 9 is open, then no problem will occur. However, if the temperature is low, then various problems occur. For example, the engine is not warmed up rapidly. The heater does not work well. Immediately after the restarting, the fan generates loud noise.
Accordingly, an improvement over the viscous fluid coupling shown in FIG. 6 has been proposed. This improved coupling is shown in FIG. 7 (which corresponds to U.S. patent application Ser. No. 07/017,458 now U.S. Pat. No. 4,784,247), where a second reservoir 6' is formed in the rear of the fluid chamber 5. When the coupling is not in operation, some of the fluid stays in the second reservoir 6' and so the liquid level h (see FIG. 8) drops accordingly.
In the conventional viscous fluid coupling shown in FIG. 6, when it is not in operation, the level h of the viscous fluid in the chamber 5 is high as shown in FIG. 8. This fluid in the chamber 5 transmits torque from the input. Therefore, the fan turns at a high speed. At low temperatures, loud noise is produced.
In the viscous fluid coupling shown in FIG. 7, the fluid chamber 5 is limited to a narrow area. This makes it impossible to control the coupling at will. As a result, the performance obtained heretofore is sacrificed.